Aluminum alloy extruded product exhibiting excellent surface properties, method of manufacturing the same, heat exchanger multi-port tube, and method of manufacturing heat exchanger including the multi-port tube

ABSTRACT

An aluminum alloy extruded product exhibiting excellent surface properties, comprising 0.8 to 1.6% of Mn and 0.4 to 0.8% of Si at a ratio of Mn content to Si content (Mn %/Si %) of 0.7 to 2.4, with the balance being Al and inevitable impurities, the number of intermetallic compounds with a diameter (circle equivalent diameter) of 0.1 to 0.9 μm dispersed in a matrix being 2×10 5  or more per square millimeter. The aluminum alloy extruded product allows extrusion of a thin multi-port tube at a high limiting extrusion rate, prevents deposits from adhering to the surface of the extruded tube, and may be suitably used as a constituent member for an aluminum alloy automotive heat exchanger.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aluminum alloy extruded product exhibiting excellent surface properties, a method of manufacturing the same, a heat exchanger multi-port tube, and a method of manufacturing a heat exchanger including the multi-port tube.

2. Description of Related Art

As a constituent member for an automotive heat exchanger such as an evaporator and a condenser, an aluminum alloy which has a reduced weight and exhibits excellent thermal conductivity has been generally used. In the manufacture of automotive heat exchangers, an aluminum alloy tube (hereinafter called “tube”), such as an aluminum alloy extruded flat multi-port tube (hereinafter called “multi-port tube”) having a plurality of hollow portions divided by a plurality of partitions, is used as the material for a working fluid passage. After applying a fluoride-type flux to the surface of the multi-port tube, the multi-port tube and other members such as a fin material are assembled into a specific structure and joined by brazing in a heating furnace containing inert gas.

In recent years, in order to reduce the fuel consumption of automobiles from the viewpoint of reducing the environmental impact, the weight of heat exchangers has been reduced. Along with this trend, the thickness of the tube has been increasingly reduced. An attempt has also been made to reduce the cross-sectional area of the tube. In this case, since the extrusion ratio (cross-sectional area of container/cross-sectional area of extruded product) of the multi-port tube reaches several hundred to several thousand, a pure Al material exhibiting excellent extrudability has been used for the multi-port tube. It is expected that the weight of exchangers and the thickness of the tube will be more and more reduced. Therefore, it is necessary to increase the strength of the tube.

It is effective to add Si, Cu, Mn, Mg, and the like in order to increase the strength of the tube. On the other hand, when the Mg content of the brazing target material exceeds 0.2%, a fluoride-type flux containing potassium fluoroaluminate which is melted during heating reacts with Mg in the material to produce compounds such as MgF₂ and KMgF₃. This reduces the activity of the flux, whereby brazeability deteriorates. The operating temperature of a heat exchanger using a carbon dioxide refrigerant is as high as about 150° C. As a result, intergranular corrosion susceptibility significantly increases when Cu is contained in the material. Therefore, Si and Mn must be necessarily added in order to increase the strength of the tube.

In an alloy containing Mn and Si at high concentrations, Mn and Si dissolved in the matrix increase the deformation resistance of the alloy. For example, when the extrusion ratio reaches several hundred to several thousand such as when manufacturing the multi-port tube, the alloy exhibits significantly inferior extrudability in comparison with a pure Al material. In this case, extrudability is evaluated using the ram pressure necessary for extrusion and the maximum extrusion rate at which the partition wall of the hollow portion of the multi-port tube is completely formed (i.e. limiting extrusion rate) as indices. A material which requires a high ram pressure or exhibits a low limiting extrusion rate is determined to have poor extrudability. An alloy containing Mn and Si at high concentrations requires a ram press higher than that of a pure Al material, whereby the die tends to break or wear. Moreover, productivity decreases due to a decrease in the limiting extrusion rate.

As a method for improving the extrudability of an aluminum alloy containing Mn and Si, a method has been proposed in which the amount of solute elements dissolved in the matrix is decreased by performing homogenization in which high-temperature heat treatment and low-temperature heat treatment are combined, thereby decreasing deformation resistance (see JP-A-11-335764). However, extrudability is not necessarily sufficiently improved when extruding a tube such as a thin multi-port tube. Therefore, a further improvement is required.

It was found that a phenomenon occurs in which an aluminum alloy is deposited in the shape of a film on the bearing of the die during extrusion and the deposit adheres to the surface of the extruded tube. A fluoride-type flux is applied to the surface of the extruded tube before brazing by roll coating or the like. In this case, the portion to which the deposit adheres is not provided with the flux. As a result, a brazing failure occurs in the portion which is not provided with the flux. There may be a case where potassium fluorozincate is applied as a flux and Zn produced by the subsequent brazing is diffused in the thickness direction and allowed to function as a sacrificial corrosion protection layer, In this case, a Zn diffusion layer is not formed in the portion which is not provided with the flux, whereby the corrosion protection performance cannot be ensured.

The film-shaped deposit on the bearing of the die is increased in thickness and amount during continuous extrusion. The deposit is finally removed from the bearing and adheres to the surface of the extruded tube. The deposition, removal, and adhesion process then repeatedly occurs. As a result, the deposit adheres to the surface of the extruded tube at specific intervals.

SUMMARY OF THE INVENTION

The present invention was achieved after further experiments and investigations conducted on the relationship among the alloy composition, heat treatment of an unextruded ingot, and extrudability in an attempt to improve the extrudability of an aluminum alloy to which Mn and Si are added to obtain high strength and to solve the problem in which the deposit adheres to the surface of the extruded tube. Accordingly, an object of the present invention is to provide an aluminum alloy extruded product exhibiting excellent surface properties which exhibits improved strength and excellent extrudability, allows extrusion of a thin multi-port tube at a high limiting extrusion rate, prevents the deposit from adhering to the surface of the extruded tube, and may be suitably used as a constituent member for an aluminum alloy automotive heat exchanger, and a method of manufacturing the same.

In order to achieve the above object, a first aspect of the present invention provides an aluminum alloy extruded product exhibiting excellent surface properties, comprising 0.8 to 1.6% (mass %; hereinafter the same) of Mn and 0.4 to 0.8% of Si at a ratio of Mn content to Si content (Mn %/Si %) of 0.7 to 2.4, with the balance being Al and inevitable impurities, the number of intermetallic compounds with a diameter (circle equivalent diameter; hereinafter the same) of 0.1 to 0.9 μm dispersed in a matrix being 2×10⁵ or more per square millimeter.

This aluminum alloy extruded product may further comprise 0.05% or less of Cu.

This aluminum alloy extruded product may further comprise 0.2% or less of Mg.

This aluminum alloy extruded product may further comprise 0.3% or less of Ti.

A second aspect of the present invention provides a heat exchanger multi-port tube comprising the above aluminum alloy extruded product.

A third aspect of the present invention provides a method of manufacturing an aluminum alloy extruded product exhibiting excellent surface properties, the method comprising: melting and casting an aluminum alloy having the above composition to obtain an ingot; subjecting the ingot to homogenization which includes a first-stage heat treatment in which the ingot is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the ingot is cooled to 400 to 500° C. at an average temperature decrease rate of 20 to 60° C./h and maintained at that temperature for three hours or more; heating the ingot at 480 to 560° C.; and extruding the ingot.

A fourth aspect of the present invention provides a method of manufacturing a aluminum alloy extruded product exhibiting excellent surface properties, the method comprising: melting and casting an aluminum alloy having the above composition to obtain an ingot; subjecting the ingot to homogenization which includes a first-stage heat treatment in which the ingot is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the ingot is cooled to room temperature, heated to 400 to 500° C. at an average temperature increase rate of 20 to 60° C./h, and maintained at that temperature for three hours or more; heating the ingot at 480 to 560° C.; and extruding the ingot.

A fifth aspect of the present invention provides a method of manufacturing a heat exchanger comprising extruding a heat exchanger multi-port tube using the above method, and joining the multi-port tube to a heat exchanger by brazing.

According to the present invention, an aluminum alloy extruded product exhibiting excellent surface properties which exhibits improved strength and excellent extrudability, allows extrusion of a thin multi-port tube at a high limiting extrusion rate, prevents the deposit from adhering to the surface of the extruded tube, and may be suitably used as a constituent member for an aluminum alloy automotive heat exchanger, a method of manufacturing the same, a heat exchanger multi-port tube made of the aluminum alloy extruded product, and a method of manufacturing a heat exchanger including the multi-port tube can be provided.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of an aluminum alloy flat multi-port tube extruded in the examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT

The meanings and the reasons for limitations of the alloy components of the aluminum alloy extruded product according to the present invention are given below. Mn and Si are dissolved in the matrix during heating for brazing to improve the strength of the alloy. The Mn content is preferably 0.8 to 1.6%, and the Si content is preferably 0.4 to 0.8%. If the content of Mn and Si is greater than the upper limit, extrudability deteriorates to a large extent to impair the strength improvement effect. If the content of Mn and Si is less than the lower limit, a sufficient strength cannot be obtained.

The ratio of the Mn content to the Si content (Mn mass %/Si mass %) is preferably 0.7 to 2.4. If the ratio of the Mn content to the Si content is within this range, Mn and Si dissolved in the matrix during casting of the alloy can be mainly precipitated as an Al—Mn—Si intermetallic compound during homogenization of the by ingot, whereby the solid solubility in the matrix can be minimized. The dispersion state in which a number of minute Al—Mn—Si intermetallic compounds are precipitated reduces the deformation resistance of the alloy during hot extrusion performed after homogenization heat treatment, whereby the extrudability of the alloy can be improved.

If the ratio “Mn %/Si %” is less than 0.7, since Si is contained in the alloy in an amount exceeding the range of the ratio “Mn %/Si %” which can minimize the solid solubility of Mn and Si in the matrix, Si remains dissolved in the matrix after the homogenization heat treatment, whereby the deformation resistance of the alloy during the subsequent hot extrusion is not reduced. As a result, the extrudability of the alloy cannot be improved. If the ratio “Mn %/Si %” exceeds 2.4, since Mn is contained in the alloy in an amount exceeding the range of the ratio “Mn %/Si %” which can minimize the solid solubility of Mn and Si in the matrix, Mn remains dissolved in the matrix after the homogenization heat treatment, whereby the deformation resistance of the alloy during the subsequent hot extrusion is not reduced. As a result, the extrudability of the alloy cannot be improved.

The Cu content is preferably limited to 0.05% or less. This reduces intergranular corrosion during use of an automotive heat exchanger manufactured by brazing the aluminum alloy extruded product according to the present invention. If the Cu content exceeds 0.05%, since the operating temperature of a heat exchanger using carbon dioxide as a refrigerant becomes as high as about 150° C., Al—Mn compounds and the like are significantly precipitated at the boundaries, whereby intergranular corrosion susceptibility increases.

Mg improves the strength of the alloy when contained in an amount of 0.2% or less. Moreover, when manufacturing an automotive heat exchanger by brazing using a fluoride-type flux contain potassium fluoroaluminate, excellent brazeability can be stably obtained. If the Mg content exceeds 0.2%, when manufacturing an automotive heat exchanger by brazing, a fluoride-type flux containing potassium fluoroaluminate which is melted during heating for brazing reacts with Mg in the material to produce compounds such as MgF₂ and KMgF₃. This reduces the activity of the flux, whereby brazeability deteriorates. Moreover, the extrudability of the alloy decreases when the Mg content exceeds 0.2%.

Ti form a high-Ti-concentration area and a low-Ti-concentration area in the alloy. These areas are alternately distributed in layers in the direction of the thickness of the material. Since the low-Ti-concentration area is preferentially corroded in comparison with the high-Ti-concentration area, corrosion occurs in layers. This prevents corrosion from proceeding in the direction of the thickness of the material. As a result, pitting corrosion resistance and intergranular corrosion resistance are improved. Moreover, the strength of the material at room temperature and a high temperature is improved by adding Ti. The Ti content is preferably 0.06 to 0.3%. If the Ti content is less than 0.06%, the effect is insufficient. If the Ti content exceeds 0.3%, coarse compounds are produced during casting, whereby workability is impaired.

Fe is contained as an inevitable impurity. The Fe content is preferably limited to about 0.7% or less, and still more preferably 0.3% or less. When adding B aiming at ingot grain refinement or the like, the B content is preferably about 0.01% or less. Impurities such as Cr, Zr, Ni, and Zn may be contained in the alloy in an amount of 0.25% or less in total.

In the aluminum alloy extruded product according to the present invention, it is important that intermetallic compounds with a diameter (circle equivalent diameter) of 0.1 to 0.9 μm be dispersed in the matrix in a number of 2×10⁵ or more per square millimeter (mm²). These intermetallic compounds are mainly Al—Mn—Si intermetallic compounds. The above dispersion structure is obtained by homogenizing an unextruded ingot (billet), which reduces adhesion of the deposit to the surface of the aluminum alloy extruded product and improves the strength of the aluminum alloy extruded product after heating for brazing. Specifically, the extruded aluminum alloy is deposited on the bearing of the die in the shape of a film. When extruding a billet in which the above intermetallic compounds are dispersed, since the surface of the film-shaped deposit formed on the bearing of the die is continuously scraped off by the dispersed minute intermetallic compounds during extrusion, the deposit is formed in the shape of a thin uniform film. Since the deposit is maintained in the shape of a thin uniform film during continuous extrusion, removal of the deposit is prevented. As a result, adhesion of the deposit to the surface of the aluminum alloy extruded product is significantly reduced. Since the deposit is maintained in the shape of a thin uniform film, the extruded product is provided with excellent surface properties to exhibit a gloss.

The extruded tube is attached to a heat exchanger (e.g. automotive heat exchanger) and joined by brazing. In this case, since the Al—Mn—Si intermetallic compounds dispersed in the matrix are redissolved in the matrix, the strength of the tube after joining by brazing is improved due to solid solution hardening. Since the operating temperature is as high as about 150° C. when using carbon dioxide as a refrigerant, the aluminum alloy extruded product is required to exhibit creep strength. Since Mn and Si (solute elements) are redissolved in the matrix after joining by brazing, these elements hinder the motion of dislocation in the matrix to improve the creep strength of the aluminum alloy extruded product.

The aluminum alloy extruded product according to the present invention is manufactured by melting an aluminum alloy having the above composition, casting the aluminum alloy by semicontinuous casting or the like to obtain an ingot (billet), and homogenizing and hot-extruding the ingot. A structure in which the above intermetallic compounds are dispersed is obtained by specifying the homogenization conditions, whereby adhesion of a deposit to the surface of the aluminum alloy extruded product is reduced, and the strength of the aluminum alloy extruded product is improved after heating for brazing. Moreover, an improved hot extrudability is obtained by combining specific homogenization conditions and hot extrusion conditions.

It is preferable to perform homogenization which includes a first-stage heat treatment in which the billet is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the billet is cooled to 400 to 500° C. at an average temperature decrease rate of 20 to 60° C./h and maintained at that temperature for three hours or more. Homogenization may be performed which includes a first-stage heat treatment in which the billet is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the billet is cooled to room temperature, heated to 400 to 500° C. at an average temperature increase rate of 20 to 60° C./h, and maintained at that temperature for three hours or more.

Coarse crystals formed during casting/solidification are decomposed, granulated, or redissolved during the first-stage heat treatment in which the billet is maintained at 550 to 650° C. for two hours or more. If the temperature is less than 550° C., the above reaction proceeds to only a small extent. The rate of reaction increases as the homogenization temperature becomes higher. On the other hand, local melting occurs when the homogenization temperature is too high. Therefore, the upper limit is preferably set at 650° C. The temperature range of the first-stage heat treatment is still more preferably 580 to 620° C. The reaction proceeds to a larger extent as the treatment time increases. Therefore, it is preferable to set the treatment time at 10 hours or more. On the other hand, a further effect cannot obtained even if the treatment is performed for more than 24 hours. This is disadvantageous from the viewpoint of cost. Therefore, the treatment time is preferably 10 to 24 hours.

The first-stage heat treatment performed at a high temperature is effective for decomposing, granulating, or redissolving coarse crystals formed during casting/solidification. On the other hand, the first-stage heat treatment promotes dissolution of Mn and Si (solute elements) in the matrix. If the solid solubility of these solute elements in the matrix is high, the moving speed of dislocation in the matrix decreases, whereby the deformation resistance of the aluminum alloy increases. Therefore, the extrudability of the aluminum alloy decreases when the aluminum alloy is hot-extruded after homogenization including only the first-stage heat treatment. In the present invention, the second-stage heat treatment is performed after the first-stage heat treatment at a temperature lower than that of the first-stage heat treatment to precipitate Mn and Si dissolved in the matrix, whereby the solid solubility of Mn and Si is decreased. This reduces the deformation resistance of the aluminum alloy, whereby the extrudability of the aluminum alloy is improved.

The second-stage heat treatment is preferably performed at 400 to 500° C. for three hours or more. If the temperature is less than 400° C., only a small amount of Al—Mn—Si intermetallic compounds precipitate, whereby the effect of decreasing the deformation resistance becomes insufficient. If the temperature exceeds 500° C., the intermetallic compounds precipitate to only a small extent, whereby the effect of decreasing the deformation resistance becomes insufficient. If the treatment time is less than three hours, since precipitation does not sufficiently proceed, the effect of deceasing the deformation resistance becomes insufficient. The reaction proceeds to a larger extent as the treatment time increases. On the other hand, a further effect cannot be obtained even if the treatment is performed for more than 24 hours. This is disadvantageous from the viewpoint of cost. The treatment time is still more preferably 5 to 15 hours.

In order to achieve the above effects during homogenization, it is important to control the temperature decrease rate from the first-stage heat treatment temperature to the second-stage heat treatment temperature (the temperature increase rate from room temperature to the second-stage heat treatment temperature when the billet is cooled to room temperature after the stage heat treatment) in order to precipitate Mn and Si dissolved in the matrix to decrease the solid solubility of Mn and Si and to achieve the above dispersion state of the intermetallic compounds. The average temperature decrease rate from the first-stage heat treatment temperature to the second-stage heat treatment temperature is preferably 20 to 60° C./h. If the average temperature decrease rate is less than 20° C./h, intermetallic compounds are grown to a large extent due to the progress of precipitation, whereby it is difficult to obtain a structure in which intermetallic compounds with a diameter of 0.1 to 0.9 μm are dispersed in a number of 2×10⁵ or more per square millimeter. Moreover, it is not economical because the treatment requires time. If the average temperature decrease rate exceeds 60° C./h, the temperature distribution of the billet becomes nonuniform, whereby precipitation tends to become nonuniform. It is also preferable that the average temperature increase rate to the first-stage heat treatment temperature and the average temperature decrease rate from the second-stage heat treatment temperature to 300° C. be 20 to 60° C./h.

When the billet is cooled to room temperature after the first-stage heat treatment and then heated to the second-stage heat treatment temperature, the average temperature increase rate is preferably 20 to 60° C./h. If the average temperature increase rate is less than 20° C./h, since precipitated intermetallic compounds are grown to a large extent, the number of intermetallic compounds is decreased, whereby the above intermetallic compound dispersion structure may not be obtained. Moreover, it is not economical because heating requires time. If the average temperature increase rate exceeds 60° C./h, it is difficult to obtain the above intermetallic compound dispersion structure since precipitation does not proceed. It is also preferable that the average temperature decrease rate from the second-stage heat treatment temperature to 300° C. be 20 to 60° C./h.

In the present invention, the solid solubility of the solute elements in the matrix is decreased by homogenizing the billet by combining the above specific high-temperature heat treatment and low-temperature heat treatment. This reduces the deformation resistance of the aluminum alloy during the subsequent hot extrusion, whereby the extrudability of the aluminum alloy can be improved. The heating temperature of the billet before hot extrusion is preferably 480 to 560° C. If the heating temperature exceeds 560° C., the precipitate mainly containing Al—Mn—Si intermetallic compounds formed during homogenization is redissolved to increase the solid solubility in the matrix. This results in an increase in deformation resistance during hot extrusion, whereby the extrudability of the aluminum alloy is decreased. If the heating temperature is less than 480° C., deformation resistance is increased due to too low a temperate, whereby the extrudability of the aluminum alloy is decreased. The heating temperature is still more preferably 480 to 530° C. The holding time at the above heating temperature is preferably 30 minutes or less. If the holding time exceeds 30 minutes, the intermetallic compounds precipitated during homogenization are redissolved to increase the solid solubility in the matrix. This results in an increase in deformation resistance during hot extrusion, whereby the extrudability of the aluminum alloy is decreased. The holding time is still more preferably 10 minutes or less.

The aluminum alloy extruded product according to the present invention has been described above taking a tube as an example. Note that the extrusion shape is not particularly limited. The extrusion shape is appropriately selected depending on the application such as the form of the heat exchanger. Multi-port tubes of various shapes may be extruded using a porthole die. When using the aluminum alloy extruded product as a working fluid passage material for a heat exchanger, the aluminum alloy extruded product and other constituent members (e.g. fin material and header material) are assembled and integrally joined by brazing. An automotive heat exchanger in which the working fluid passage is formed using the above multi-port tube exhibits excellent corrosion resistance and exhibits excellent durability even under a severe corrosive environment.

EXAMPLES

The present invention is described below by way of examples and comparative examples to demonstrate the effects of the present invention. Not that these examples illustrate one aspect of the present invention, and should not be construed as limiting the present invention.

Example 1 and Comparative Example 1

An aluminum alloy having the composition shown in Table 1 was melted and cast by semicontinuous casting to obtain a billet. The resulting billet was homogenized. The billet was homogenized by increasing the temperature of the billet to a first-stage heat treatment temperature of 600° C. at an average temperature increase rate of 50° C./h, maintaining the billet at the first-stage heat treatment temperature for 15 hours, decreasing the temperature of the billet to a second-stage heat treatment temperature of 450° C. at an average temperature decrease rate of 50° C./h, maintaining the billet at the second-stage heat treatment temperature for 10 hours, and decreasing the temperature of the billet from the second-stage heat treatment temperature to 300° C. at an average temperature decrease rate of 50° C./h. After homogenization, the billet was heated at 510° C. for eight minutes and hot-extruded to obtain a multi-port tube having a shape shown in FIG. 1. The resulting multi-port tube was used as a test specimen.

The extrudability of the aluminum alloy during hot extrusion was evaluated according to the following method. Likewise, the number of deposit portions adhering to the surface of the extruded multi-port tube was calculated, and the gloss of the multi-port tube was observed. The distribution of intermetallic compounds precipitated and dispersed in the matrix was also determined. The multi-port tube was subjected to joining by brazing, and brazeability, tensile strength after heating for brazing, and intergranular corrosion susceptibility were evaluated. The results are shown in Table 2. In Tables 1 and 2, values outside the conditions according to the present invention are underlined.

Evaluation of extrudability: The limiting extrusion rate (i.e. the maximum extrusion rate at which the partition wall of the hollow portion of the extruded multi-port tube (see FIG. 1) is completely formed) was taken as the extrudability index. The limiting extrusion rate indicates the ratio of the limiting extrusion rate of the aluminum alloy to the limiting extrusion rate of a known alloy (see Table 1) (ratio when the limiting extrusion rate of the known alloy is 1.0). An aluminum alloy with a limiting extrusion rate ratio of 0.9 or more was indicated as “Excellent”, an aluminum alloy with a limiting extrusion rate ratio of 0.8 or more and less than 0.9 was indicated as “Good”, an aluminum alloy with a limiting extrusion rate ratio of 0.7 or more and less than 0.8 was indicated as “Fair”, and an aluminum alloy with a limiting extrusion rate ratio of less than 0.7 was indicated as “Bad”.

Measurement of number of deposit portions adhering to surface and observation of gloss of surface of extruded product: A portion to which foreign matter adhered was detected using an eddy current test, and the number of portions of the surface of the extruded product to which an aluminum alloy deposit adhered was determined to calculate of the number of deposit portions per unit length of the extruded product. The gloss of the surface of the extruded product was evaluated by naked eye observation, and was also taken as the index of adhesion of deposit to the surface of the extruded product.

Evaluation of distribution (dispersion structure) of intermetallic compounds: the cross-sectional microstructure of the extruded product was observed, and the number of precipitated intermetallic compounds with a diameter (circle equivalent diameter) of 0.1 to 0.9 μm was determined by image analysis.

Measurement of tensile strength after heating for brazing: The multi-port tube obtained by extrusion was heat-treated at 600° C. for three minutes in a nitrogen atmosphere as simulated heating for brazing, cooled at an average temperature decrease rate of 50 to 250° C./min, and subjected to a tensile test to determine the strength of the multi-port tube. A multi-port tube with a tensile strength of 110 MPa or more was determined to have a sufficient tensile strength.

Evaluation of brazeability: A fluoride-type flux containing potassium fluoroaluminate was applied to the surface of the extruded multi-port tube in an amount of 10 g/m². The multi-port tube and a fin were assembled and joined by brazing by heat-treating the product at 600° C. for three minutes in a nitrogen atmosphere and cooling the product at an average temperature decrease rate of 50 to 250° C./min. The joining state of the multi-port tube with the fin was then observed. A case where the multi-port tube and the fin were sufficiently joined was indicated as “Good”, and a case where the multi-port tube and the fin were not sufficiently joined was indicated as “Bad”.

Evaluation of intergranular corrosion susceptibility: In order to simulate the use at 150° C., the multi-port tube subjected to the above simulated heating for brazing was heat-treated at 150° C. for 120 hours and immersed for 24 hours in a solution prepared by adding 10 ml/l HCl to a 30 g/l NaCl aqueous solution. The cross section of the multi-port tube was then observed. A multi-port tube in which intergranular corrosion did not occur was indicated as “Good”, and a multi-port tube in which intergranular corrosion occurred was indicated as “Bad”. TABLE 1 Composition (mass %) Alloy Si Fe Cu Mn Mg Mn/Si Invention A 0.6 0.2 0 1.2 0 2 B 0.5 0.2 0 1 0.1 2 C 0.45 0.2 0 1 0.15 2.2 D 0.7 0.2 0 1.4 0.1 2 E 0.8 0.2 0 0.8 0 1 Comparison F 1.5 0.2 0 1.9 0 1.3 G 0.05 0.2 0 0.1 0 2 H 0.6 0.2 0.3 1.2 0 2 I 0.6 0.2 0 1.2 0.6 2 J 0.05 0.2 0.4 0.1 0 2

TABLE 2 Number of intermetallic compounds with Number of diameter of Limiting Tensile Intergranular deposit Test 0.1 to 0.9 μm extrusion rate strength corrosion portions Surface specimen Alloy (10⁵/mm²) ratio Brazeability (MPa) Susceptibility (/10000 m) gloss 1 A 3.2 1.0 (Excellent) Good 114 Good 0 Good 2 B 3.5 0.95 (Excellent) Good 120 Good 0 Good 3 C 3.1 0.9 (Excellent) Good 110 Good 0 Good 4 D 3.8 0.8 (Good) Good 130 Good 0 Good 5 E 4.1 0.85 (Good) Good 110 Good 0 Good 6 F 5.0 0.4 (Bad) Good 145 Good 0 Good 7 G 0.5 1.0 (Excellent) Good 68 Good 2.4 Bad 8 H 2.8 0.7 (Fair) Good 122 Bad 0 Good 9 I 3.1 0.6 (Fair) Bad 168 Good 0 Good 10 J 0.5 1.0 (Excellent) Good 72 Bad 3.6 Bad

As shown in Table 2, the test specimens 1 to 5 according to the present invention exhibited excellent extrudability, did not show adhesion of deposit to the surface, and exhibited excellent brazeability, intergranular corrosion resistance, and strength. On the other hand, the test specimens 6 to 9 and the test specimen 10 (known alloy) were inferior in at least one of extrudability, adhesion of deposit, strength, brazeability, and intergranular corrosion resistance.

Comparative Example 2

An aluminum alloy having the composition A shown in Table 1 was melted and cast by semicontinuous casting to obtain a billet. The resulting billet was homogenized under the conditions shown in Table 3. The billet was homogenized by increasing the temperature of the billet to a first-stage heat treatment temperature at an average temperature increase rate of 50° C./h, maintaining the billet at the first-stage heat treatment temperature, decreasing the temperature of the billet to a second-stage heat treatment temperature, maintaining the billet at the second-stage heat treatment temperature, and decreasing the temperature of the billet to 300° C. at an average temperature decrease rate of 50° C./h. Table 3 shows the first-stage heat treatment temperature, the average temperature decrease rate from the first-stage heat treatment temperature to the second-stage heat treatment temperature, and the second-stage heat treatment temperature. After homogenization, the billet was hot-extruded under the conditions shown in Table 3 to obtain a multi-port tube shown in FIG. 1. The resulting multi-port tube was used as a test specimen.

The extrudability of the aluminum alloy during hot extrusion was evaluated in the same manner as in Example 1. Likewise, the number of deposit portions adhering to the surface of the extruded multi-port tube was calculated, and the gloss of the multi-port tube was observed. The distribution of intermetallic compounds precipitated and dispersed in the matrix was also determined. The multi-port tube was subjected to joining by brazing, and brazeability, tensile strength after heating for brazing, and intergranular corrosion susceptibility were evaluated. The results are shown in Table 4. In Tables 3 and 4, values outside the conditions according to the present invention are underlined. TABLE 3 Homogenization First-stage Average Second-stage Extrosion heat treatment temperature heat treatment Billet Test (temperature decrease (temperature Billet heating heating time specimen Alloy (° C.) × time (h)) rate (° C./h) (° C.) × time (h)) temperature (° C.) (min) 11 A 530 × 15 50 450 × 10 510 8 12 A 600 × 15 50 530 × 10 510 8 13 A 600 × 15 50 450 × 1  510 8 14 A 600 × 15 15 450 × 10 510 8 15 A 600 × 15 50 450 × 10 580 35

TABLE 4 Number of intermetallic compounds with Number of diameter of Limiting Tensile Intergranular deposit Test 0.1 to 0.9 μm extrusion rate strength corrosion portions Surface specimen Alloy (10⁵/mm²) ratio Brazeability (MPa) susceptibility (/10000 m) gloss 11 A 2.5 0.75 (Fair) Good 114 Good 0.3 Fair 12 A 2.1 0.7 (Fair) Good 114 Good 0.3 Fair 13 A 1.6 0.7 (Fair) Good 115 Good 0.3 Fair 14 A 3.1 0.75 (Fair) Good 113 Good 0.3 Fair 15 A 1.5 0.7 (Fair) Good 114 Good 0.3 Fair

As shown in Table 4, the test specimens 11 to 15 homogenized under the conditions outside the conditions according to the present invention were inferior in at least one of extrudability, number of deposit portions, strength, brazeability, and intergranular corrosion resistance.

Example 2 and Comparative Example 3

An aluminum alloy containing 0.6% of Si, 0.2% of Fe, and 1.0% of Mn (Mn %/Si %: 1.7) was melted and cast by semicontinuous casting to obtain a billet. The resulting billet was homogenized under the conditions shown in Table 5. The billet was homogenized by increasing the temperature of the billet to a first-stage heat treatment temperature at an average temperature increase rate of 50° C./h, maintaining the billet at the first-stage heat treatment temperature, decreasing the temperature of the billet to room temperature, increasing the temperature of the billet to a second-stage heat treatment temperature, maintaining the billet at the second-stage heat treatment temperature, and decreasing the temperature of the billet to 300° C. at an average temperature decrease rate of 50° C./h. Table 5 shows the first-stage heat treatment temperature, the second-stage heat treatment temperature, and the average temperature increase rate from room temperature to the second-stage heat treatment temperature. After homogenization, the billet was hot-extruded under the conditions shown in Table 5 to obtain a multi-port tube shown in FIG. 1. The resulting multi-port tube was used as a test specimen.

The extrudability of the aluminum alloy during hot extrusion was evaluated in the same manner as in Example 1. Likewise, the number of deposit portions adhering to the surface of the extruded multi-port tube was calculated, and the gloss of the multi-port tube was observed. The distribution of intermetallic compounds precipitated and dispersed in the matrix was also determined. The multi-port tube was subjected to joining by brazing, and brazeability, tensile strength after heating for brazing, and intergranular corrosion susceptibility were evaluated. The results are shown in Table 6. In Tables 5 and 6, values outside the conditions according to the present invention are underlined. TABLE 5 Homogenization First-stage Average Second-stage Extrusion heat treatment temperature heat treatment Billet heating Test (temperature increase (temperature temperature Billet heating specimen (° C.) × time (h)) rate (° C./h) (° C.) × time (h)) (° C.) time (min) 16 600 × 15 50 450 × 10 510 8 17 530 × 15 50 450 × 10 510 8 18 600 × 15 50 530 × 10 510 8 19 600 × 15 50 380 × 10 510 8 20 600 × 15 15 450 × 10 510 8 21 600 × 15 50 450 × 10 580 20

TABLE 6 Number of intermetallic Limiting Tensile Intergranular Number of Test compounds with diameter of extrusion rate strength corrosion deposit portions specimen 0.1 to 0.9 μm (10⁵/mm²) ratio Brazeability (MPa) susceptibility (/10000 m) Surface gloss 16 3.0 1.0 (Excellent) Good 113 Good 0 Good 17 2.5 0.75 (Fair) Good 113 Good 0.2 Fair 18 2.0 0.75 (Fair) Good 114 Good 0.3 Fair 19 1.5 0.7 (Fair) Good 116 Good 0.3 Fair 20 3.0 0.75 (Fair) Good 112 Good 0.4 Fair 21 1.4 0.7 (Fair) Good 113 Good 0.4 Fair

As shown in Table 6, the test specimen 16 according to the present invention exhibited excellent extrudability, did not show adhesion of deposit to the surface, and exhibited excellent brazeability, intergranular corrosion resistance, and strength. On the other hand, the test specimens 17 to 21 were inferior in at least one of extrudability, adhesion of deposit, strength, brazeability, and intergranular corrosion resistance.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

1. An aluminum alloy extruded product exhibiting excellent surface properties, comprising 0.8 to 1.6% (mass %; hereinafter the same) of Mn and 0.4 to 0.8% of Si at a ratio of Mn content to Si content (Mn %/Si %) of 0.7 to 2.4, with the balance being Al and inevitable impurities, the number of intermetallic compounds with a diameter (circle equivalent diameter; hereinafter the same) of 0.1 to 0.9 μm dispersed in a matrix being 2×10⁵ or more per square millimeter.
 2. The aluminum alloy extruded product according to claim 1, further comprising 0.05% or less of Cu.
 3. The aluminum alloy extruded product according to claim 1, further comprising 0.2% or less of Mg.
 4. The aluminum alloy extruded product according to claim 1, further comprising 0.30% or less of Ti.
 5. A heat exchanger multi-port tube comprising the aluminum alloy extruded product according to claim
 1. 6. A method of manufacturing an aluminum alloy extruded product exhibiting excellent surface properties, the method comprising: melting and casting an aluminum alloy having the composition according to claim 1 to obtain an ingot; subjecting the ingot to homogenization which includes a first-stage heat treatment in which the ingot is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the ingot is cooled to 400 to 500° C. at an average temperature decrease rate of 20 to 60° C./h and maintained at that temperature for three hours or more; heating the ingot at 480 to 560° C.; and extruding the ingot.
 7. A method of manufacturing an aluminum alloy extruded product exhibiting excellent surface properties, the method comprising: melting and casting an aluminum alloy having the composition according to claim 1 to obtain an ingot; subjecting the ingot to homogenization which includes a first-stage heat treatment in which the ingot is maintained at 550 to 650° C. for two hours or more and a second-stage heat treatment in which the ingot is cooled to room temperature, heated to 400 to 500° C. at an average temperature increase rate of 20 to 60° C./h, and maintained at that temperature for three hours or more; heating the ingot at 480 to 560° C.; and extruding the ingot.
 8. A method of manufacturing a heat exchanger comprising extruding a heat exchanger multi-port tube using the method according to claim 6, and joining the multi-port tube to a heat exchanger by brazing. 